Heat Units: use SWAT or PHU results?

[Justin Goldstein]

Colleagues,
  I had SWAT estimate the potential heat units for the land covers in my watershed when writing the .mgt files.  Those for range-grasses come up as 2070, and of course with 15% of heat units for planting (default).  I also computed the heat units using the potential heat units (PHU) calculator.  The PHU calculator came up with 1373 heat units, with 11% heat units during planting.  The weather station used in the PHU calculator is 45 miles from my watershed. 

   I see that indeed SWAT is estimating PHU based on location because the values for range grasses do vary across the different watersheds I've modeled.

  Ordinarily, this is no big deal but it actually makes a large difference in my calibration.  The 2070 heat units with 15% used for planting improves my calibration Nash-Sutcliffe by 10% relative to using the values from the PHU calculator but of course I would like the science to drive the model and not the other way around.  In effect, I would need to perform two significantly different calibrations to reconcile the issue given the divergence in results.  (E.g. I would use Penman-Monteith eT for 2070 but Priestley-Taylor for 1373).    However, 2070 heat units is a lot more than 1373.  The watershed is in Western Oklahoma, USA.  Does anyone have any advice on this matter?  Also, can anyone explain this apparently large discrepancy in values?  I fully understand and have investigated the science of "heat units."

   Thanks so very much.

 

[Jochen]

Hi Justin,

I also used the PHU program to determine plant-sepcific PHU. My problem is slighty different. Do you have a source for the days of certain plants to reach maturity?

Thanks
Jochen

[Naresh Pai]

Perhaps this document on the SWAT website may help:

http://swat.tamu.edu/media/4015/crops-typicalplanting-harvestingdates-by-states.pdf 

Naresh

[Justin Goldstein]

Naresh,
  Thanks for this good link. 
  Jochen, Srini subsquently resolved the issue; I used the default values within SWAT proper for range grasses.

  For the dates of various management operations, I relied heavily on local agricultural extension agents.

  Also, at the end of the Swat input/output file, there is an extensive documentation of all the agronomic properties of each crop/grass listed in the crop database, including the sources for all biophysical parameters used in SWAT.
 
 Hope this helps.
   -Justin

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[Justin Goldstein]

Jochen,
I think it's total quantity of days in the field.  I had a similar question regarding native grasses (in the U.S. Great Plains) and switchgrass; here was Srini's response:


"

 So we want to keep the range grass growing throughout the year, where as switch
grass grows effectively during summer...or warm period or growing period...like for
example corn takes say 120 days to grow, then the heat units are calculated just
for the 120 days, where as range is something grows throughout the year...there is
not planting/harvest etc..."

Based on that, I used the 2070.  Again, I had a slightly different question.  Mine was why does SWAT default to 2070 heat units for range grasses in my region but the PHU program indicates 1373, but I think Srini's response is helpful to both of us.

Dos this help?
-Justin
 

On Wed, Aug 22, 2012 at 3:53 AM, Jochen <Jochen....@gmx.de> wrote:

@ Naresh and Justin:

Thanks for the quick response. That helps a lot.
How do you treet winter crops? When calculating the days to maturity I would choose the whole period of time that the plant is on the field. I don't know the code behind the PHU program but is winter dormacy taken into account?
Otherwise the total days to maturity are quite high, for example winter rye is approximately 290 days on the field.

Best regards
Jochen

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[Jochen]

@ Justin,

yes, that helps.
I think I'll try the long period and have a look at the yields then. If they are far too high, I have to look for other options.

Regards,
Jochen

[Jochen]

@ Naresh and Justin:

Thanks for the quick response. That helps a lot.
How do you treet winter crops? When calculating the days to maturity I would choose the whole period of time that the plant is on the field. I don't know the code behind the PHU program but is winter dormacy taken into account?
Otherwise the total days to maturity are quite high, for example winter rye is approximately 290 days on the field.

Best regards
Jochen

Am Dienstag, 21. August 2012 15:35:11 UTC+2 schrieb Naresh Pai: